Secondary batteries are used for consumer product applications requiring low electric power such as notebook type personal computers and mobile phones, and used as the storage battery of hybrid vehicles and electric vehicles. As the secondary batteries in these applications, a lithium-ion secondary battery which is a kind of nonaqueous secondary batteries (nonaqueous electrolyte secondary batteries) is mostly used due to a high energy density thereof.
Generally, as the electrode of a nonaqueous secondary battery, an electrode including a current collector such as a metal foil and a mixture layer provided on the current collector is used, and an active material and a conductive auxiliary are held by a binder resin in the mixture layer. The electrode is usually produced by mixing and kneading an active material, a conductive auxiliary, a binder resin, and a liquid medium (solvent) to prepare a slurry composition, coating the slurry composition on one or both sides of a current collector using a transfer roll or the like, removing the liquid medium by drying to form a mixture layer, and then performing compression molding using a roll press machine or the like if necessary. As the liquid medium, a liquid medium which disperses an active material and a conductive auxiliary, and dissolves a binder resin is used.
Hitherto, as a binder resin for a nonaqueous secondary battery electrode, for example, a fluorine resin such as polyvinylidene fluoride (PVDF) is used. PVDF is widely used since PVDF has advantages that rheological properties (thixotropy) are favorable when prepared as a slurry composition, and is electrochemically stable in the positive and negative electrodes.
However, when producing an electrode, a binder resin such as PVDF is dissolved in an organic solvent such as N-methyl-2-pyrrolidone (NMP) and used. Accordingly, problems such as a high recovery cost of solvent during drying and high environmental impact become apparent.
For that reason, an attempt to replace the organic solvent with water has recently been made, and PVDF is often used in a latex state dispersed in water for a negative electrode. In addition, as a binder resin for a negative electrode, a binder resin of aqueous dispersion system such as styrene-butadiene rubber (SBR) latex or carboxymethyl cellulose (CMC) serving as a thickener is used in some cases.
However, PVDF or SBR has a problem that the binding strength thereof is low. For that reason, improvement in battery performance such as the capacity of nonaqueous secondary battery, rate characteristics, and cycle characteristics is difficult in a case in which PVDF or SBR is used as a binder resin. For example, an increase in an amount of the conductive auxiliary is effective to improve the rate characteristics affected by the ease of electron transfer. The amount of binder is required to be reduced in order to increase the amount of the conductive auxiliary in the limited space of the battery. However, when the amount of binder is reduced, the adhesive property (binding property) between the current collector and the mixture layer or the adhesive property between the active materials deteriorates, and thus the mixture layer is peeled off from the current collector or the active material is lost from the mixture layer by the repeated charge and discharge, which results in the deterioration in battery performance.
To cope with such problems, a method in which various parameters are defined for the adhesive property (binding property) with respect to the current collector, or the like has been proposed. For example, Patent Document 1 discloses a method in which the second virial coefficient of the binder composition containing a polymer having a THF gel content of 5% or less and an organic solvent having NMP as the main component, which is measured by a static light scattering method, is set to equal to or lower than a specific value, thereby defining the radius of gyration of the polymer in a specific range. In addition, Patent Document 2 discloses a method in which two kinds of liquid medium dispersions of polymers which have the most frequent particle sizes of the primary particles in specific ranges, respectively, are mixed in specific blending amounts.
In addition, since a binder resin of aqueous dispersion system is distributed in the state containing water, there is a problem in that the transportation cost thereof increases. Moreover, there is a concern that the battery performance deteriorates since a fungicide is commonly added in the binder resin of aqueous dispersion system for the purpose of mold growth suppression.
Hence, it has been demanded that the binder resin of aqueous dispersion system is provided in the form of powder, but a latex resin has a composition exhibiting a low glass transition temperature in many cases, and thus there is a problem in that polymer chains are tangled when the latex resin is once powdered, and the latex resin hardly disperses in water again. As a result thereof, it has been difficult to provide the binder resin of aqueous dispersion system in the form of powder.
Consequently, a powder form binder resin that can be used by being dissolved or dispersed in water at the time of producing an electrode is required such that PVDF powder is used by being dissolved in NMP.
On the other hand, CMC can be used by being dissolved in water at the time of producing an electrode since CMC is a water-soluble polymer. However, the quality for each supply lot is hardly stable since CMC is derived from a natural product, as a result thereof, there is a problem in that the quality of the electrode to be obtained is also hardly stable, or the like.
Consequently, a binder resin which is an unnatural product suppliable in stable quality and is soluble in water is desired.
In addition, the binder resin is required to have high battery performance as well.
To cope with such problems, a polymer having an N-vinylacetamide unit has been reported as a binder resin.
For example, Patent Document 3 discloses a resin component containing poly(N-vinylacetamide) and a copolymer of ethylene oxide (EO) and propylene oxide (PO) as a binder resin. According to this binder resin, it is regarded that the binding property, battery characteristics under an environment from a low temperature to room temperature, and the conductivity of lithium ion are excellent.
However, the copolymer of EO and PO is dissolved into the electrolyte in some cases since the EO chain or the PO chain has a molecular structure similar to the composition of the electrolyte, and thus it is concerned that the battery performance is adversely affected.
Patent Document 4 discloses a positive electrode paste for a nonaqueous battery containing poly(N-vinylacetamide) as a polymer containing a repeating structural unit having an amide structure. Poly(N-vinylacetamide) is regarded to be capable of improving the performance required to a secondary battery (especially, a nonaqueous secondary battery) such as paste stability, binding property, and electrochemical stability. In addition, dissolution into the electrolyte also hardly occurs.